A Review on the Synthesis of Manganese Oxide Nanomaterials and Their Applications on Lithium-Ion Batteries

Most recently, manganese oxides nanomaterials, including MnO and MnO 2 , have attracted great interest as anode materials in lithium-ion batteries (LIBs) for their high theoretical capacity, environmental benignity, low cost, and special properties. Up to now, manganese oxides nanostructures with excellent properties and various morphologies have been successfully synthesized. Herein, we provide an in-depth discussion of recent development of the synthesis of manganese oxides nanomaterials and their application in the field of LIBs.


Introduction
Nanomaterials, having a length scale less than 100 nm, have received increasing interest owing not only to their fundamental scientific significance but also to the potential applications that derive from their fascinating electrical, magnetic, and catalytic properties [1].Compared to bulk active electrode materials, the corresponding nanomaterials possess more excellent electrochemical activity, such as higher capacities, larger surface areas, and lower current densities, thereby, nanomaterials have wildly potential application in electrochemistry field.Manganese oxides, including MnO, MnO 2 , and Mn 3 O 4 , are intriguing composites and have been used in wastewater treatment, catalysis, sensors, supercapacitors, and alkaline and rechargeable batteries [2][3][4][5][6].Particularly, MnO and MnO 2 nanomaterials have attracted great interest as anode materials in lithium-ion batteries (LIBs) for their high theoretical capacity, low cost, environmental benignity, and special properties [7][8][9].
It is known that the phases, sizes, and morphologies of nanomaterials have great influence on the properties and applications; therefore, many research efforts have focused on rational control of phase, shape, size, and dimensionality of nanomaterials [14].Several novel and effective routes have been devoted to prepare manganese oxides nanomaterials with various shapes and excellent properties, such as hydrothermal method [15][16][17][18], sol-gel synthesis [19], wet chemical route [12,20,21], pulsed laser deposition method [22], and precursor technique [23].Moreover, lots of successes on the properties and applications of manganese oxides nanomaterials have been reported in the last few years, for example, a hydrothermal method has been used to synthesize sea urchin shaped -MnO 2 [24]; Wu et al. have prepared -MnO 2 hexagon-based layer-cake-like and intertexture-like nanoarchitectures via a hydrothermal route [25]; Liu and coworkers have found MnO 2 nanoparticleenriched poly (3,4-ethylenedioxythiophene) nanowires that could maintain high specific capacitance at high chargedischarge rates [26].Thus, it is necessary to review the development of manganese oxides nanomaterials to keep the readers abreast of the rapid development.In this paper, we review the synthesis of manganese oxides nanomaterials with various morphologies and their application on LIBs; furthermore, the future prospects have also been discussed.

Synthesis of Manganese
Oxide Nanomaterials  categories: the chain-like tunnel structure such as -, -, and -types, the sheet or layered structure such as -MnO 2 , and the 3D structure such as -type [27].The properties of MnO 2 are significantly affected by their phases and morphologies; moreover, the operating properties of LIBs also depend on the phase of MnO 2 .In this regard, a great effort has been directed toward the preparation of MnO 2 with different phases and various shapes [28].Generally, MnO 2 nanostructures could be synthesized through the oxidation of Mn 2+ , reduction of MnO 4 − , redox reactions between Mn 2+ and MnO 4 − , or direct phase transformation from other manganese oxides.
1D MnO 2 may provide the possibility of detecting the theoretical operating limits of LIBs, so various 1D MnO 2 nanomaterials have been synthesized [29,30].Chen et al. have synthesized MnO 2 with different crystal structures (-, -) and morphologies via quick precipitation of Mn 2+ and Mn 7+ in water isopropanol without using templates or surfactants [31].In a typical synthesis, MnCl 2 (0.18 g) mixed with isopropanol (50 mL) was heated to 83 ∘ C in a refluxing process, and then KMnO 4 (0.10 g) dissolved in DI water (5 mL) was added to the solution.Finally, MnO 2 nanoneedles were obtained.Singly-crystal nanowires of and -MnO 2 have been prepared in a hydrothermal procedures employing Mn 2+ with oxidizing reagents such as (NH 4 ) 2 S 2 O 8 or KMnO 4 [28,32].Ma group have used a hydrothermal method for MnO 2 nanobelts, which have narrow size dispersions and can be self-assembled into bundles [6].In a typical procedure, Mn 2 O 3 powders (2 g) were dispersed in NaOH aqueous solution (10 mol⋅dm −3 ), and then the solution was sealed and heated at 170 ∘ C for 12 h to 1 week.Sui et al. have synthesized -MnO 2 nanowires and -MnO 2 nanorods via molten salt method [10].In the synthesis, KNO 3 , NaNO 3 , and LiNO 3 are applied as the reaction media.-MnO 2 is a (2 × 2) and (1 × 1) tunnel structure, and large ions (K + ) are needed to support the framework; thus, KNO 3 is used as molten salt to prepare -MnO 2 nanomaterials (Figure 1(a)).-MnO 2 is a (1 × 1) tunnel structure, so a mixture of NaNO 3 and LiNO 3 with smaller cation is selected.Zheng et al. have prepared -MnO 2 nanotubes using MnSO 4 as reagent, PVP as morphology directing agent and NaClO 3 as oxidant [33].-MnO 2 nanotubes have been synthesized by a hydrothermal treatment of KMnO 4 in HCl solution [11].As shown in Figure 1(b), the obtained nanotubes have an average outer diameter of 100 nm and the wall thickness of 30 nm, and the length is up to several microns.It is found that the nanotubes are formed via solid nanorods involving in chemical etching process.
Compared to 1D/2D nanostructures, 3D MnO 2 hierarchical structures often produce more active sites or possess more interesting properties, so great interests have been given to well-defined MnO 2 architectures with controlled crystal structures [38].Six-branched -MnO 2 architectures have been synthesized via an aqueous chemical route without any organic templates (Figure 2(a)).The growth rate along the six-fold c-axis is faster than along the other axes for the crystal structure of -MnO 2 , which results in the elongated twinned pyramidal shape of the crystals.Consequently, the edges of the adjacent facets of the pyramid become the nucleation sites, and the sprouted branches form the central core [13].Jana et al. have used a green method for the synthesis of hierarchical flower-like Ag-doped -MnO 2 nanostructures at 300 K.As displayed in Figure 2(b), the flowery nanostructures are composed of tiny nanopetals (500 nm in diameter and 1.25 m in length) [12].Fei group have reported a controlled synthesis of hollow microspheres and microcubes of hierarchical MnO 2 superstructures using MnCO 3 crystals as the templates [39].MnCO 3 microspheres and microcubes are synthesized by the reaction of MnSO 4 and NH 4 HCO 3 , and MnO 2 hollow microspheres and microcubes are prepared by mixing KMnO 4 and the solid MnCO 3 crystals.In The legs of the multipods are 200-600 nm in width and several microns in length 2006 [36] the synthesis, a microscale Kirkendall effect is used for the synthesis of hollow microstructures.Other hierarchical architectures with novel morphologies have also been prepared [20,[40][41][42][43][44], such as -MnO 2 nanodisks assembled from nanoparticles via a novel wet chemical route [20] and MnO 2 microspheres composed of nanodisks [41].

Synthesis of MnO Nanomaterials.
MnO, a simple binary metal oxide, has an Fm-3m rock-salt structure with a lattice constant of 4.445 Å at 300 K and has attracted strong interest for its application as catalysts [45], contrast enhancement for magnetic resonance imaging (MRI) [46], and LIBs materials [47].Currently, several methods have been developed for the fabrication of MnO nanostructures with well-controlled shapes, such as nanocrystals [48], nanofibers [49], and nanosheets [50].Monodisperse MnO nanocrystals have been synthesized by thermal decomposition of manganese oleate using the hot-injection method or thermal decomposition of manganese acetate in the presence of oleic acid [51,52]

Applications of Manganese Oxide Nanomaterials on Lithium-Ion Batteries (LIBs)
Lithium-ion batteries (LIBs) are regarded as a promising rechargeable power sources for hybrid electric vehicles (HEVs) and portable electronic devices for their high specific capacity, long cycle life, and lack of memory [65].Electrode materials play an important role in the performance of LIBs.It was found that transition metal oxides nanomaterials are very appealing anode materials owing to their higher theoretical capacities than that of commercial graphite (372 mA⋅h⋅g −1 ) [66].Among them, nanoscales MnO and MnO 2 have attracted more and more attention due to the high theoretical capacities, environmentally benign, low cost, and special properties.Zhao et al. have synthesized nanoporous -MnO 2 hollow microspheres and nanocubes with high initial capacities and excellent cycle performance in LIBs.The -MnO 2 architectures provide more possibility to serve as an ideal host material for the insertion and extraction of lithium ions for the nanoporous structure.After 20 cycles, the capacities of the -MnO 2 microspheres and nanocubes are 602.1 and 656.5 mA⋅h⋅g −1 [67].Interconnected porous MnO nanoflakes have been prepared on Ni foam.The obtained nanoflakes retain a capacity of 708.4 mA⋅h⋅g −1 at the 200th chargedischarge cycle after cycling with different current densities up to 2460 mA⋅g −1 and deliver a capacity of 376.4 mA⋅h⋅g −1 at 2460 mA⋅g −1 .The special morphology of the porous MnO nanoflake affected its electrochemical property: (I) the nanomaterials have a large specific area and offer a large material/electrolyte contact area; (II) the structure can supply enough space to buffer the volume change caused by the electrochemical reaction; (III) the nanosize of flakes leads to a shortened electronic and ionic transport length [59].In addition, Chen et al. have reported the best cycle performance of MnO anode material, which deliver a capacity of 650 mA⋅h⋅g −1 after 150 cycles at 35.5 mA⋅g −1 [68].
Despite the above successes, there are still many challenges in using MnO 2 and MnO as anode materials for LIBs, such as poor cycling performance and poor electrical conductivity.It has been demonstrated that electrode materials with a deliberately designed nanostructure can partly accommodate the strains of Li + intercalation and deintercalation [62].The electrical conductivity of manganese oxide can be enhanced by mixing them with electrolytes

Conclusions
In summary, we have briefly reviewed the recent development of manganese oxide nanomaterials.In the near future, we will face some challenges in spite of the successes discussed above.For instance (I) manganese oxide nanomaterials have recently been synthesized in the labs, and they should be applied in industry.Therefore, we should explore simple and effective methods for the synthesis of manganese oxide nanomaterials with high surface areas and good dispersity, and (II) the application of manganese oxide in the field of electrochemistry is still in its infancy; moreover, some data and conclusions are controversial.Thus, the electrochemical mechanism of manganese oxide nanomaterials should be deeply understood.In short, we hope that this paper will not only show the development of manganese oxide nanomaterials but also give the readers some inspiration to explore novel routes for the synthesis of manganese oxide nanomaterials.

Table 1 :
Synthesis of MnO 2 with different morphologies.